1. Field of the Invention
The present invention relates to a ceramic tile using a sludge slag which is available from sludge separated from sewage.
Along with the popularization of the sewage system, the quantity of produced sludge such as grit and screenings in sewage is yearly increasing. Sludge has so far mostly been disposed of by burning same into ash and dumping the produced ash onto land or into the sea. However, the increasing tendency toward urbanization is making it more and more difficult to find places for dumping. It is also necessary to convert detrimental heavy metals contained in sludge into insoluble substances. The treatment of sludge is thus an important social issue at present, and the general attention is attracted to the effective utilization of sludge as a useful resource.
Typical raw materials for ceramic tiles used in building purposes include clay, agalmatolite, silica sand and feldspar. Functions of these raw materials for conventional ceramic tiles are described below.
Clay comprises essentially kaolin, and has a function of a binder. When making a formed body by means of a press, clay has a function of improving formability of the formed body. In addition, clay has a function of melting during firing to fasten aggregates to each other.
Agalmatolite has a function as an aggregate and as a binder. Agalmatolite is economically advantageous for the low cost.
Silica sand has a function as an aggregate. Silica sand is however a raw material hindering a sintering reaction.
Feldspar has a function of melting during firing to fasten aggregates to each other and a function of reducing a firing temperature. A raw material which, like feldspar, contains alkaline elements and melts during firing, thus fastening aggregates to each other and reducing the firing temperature, is called a sintering agent. Feldspar contains also aluminum oxide (Al.sub.2 O.sub.3) and silicon dioxide (SiO.sub.2) Apart from feldspar, borax (Na.sub.2 B.sub.4 O.sub.7.10H.sub.2 O), sodium silicate (Na.sub.2 SiO.sub.3) and potassium silicate (K.sub.2 O.SiO.sub.2) are employed as the sintering agents.
When manufacturing a ceramic tile from clay, silica sand and feldspar, feldspar is added in an amount within a range of from 20 to 40 wt. %, or from 20 to 50 wt. % in some cases. However, because feldspar contains alkaline elements in an amount of only from 14 to 15 wt. %, a firing temperature of over 1,200.degree. C. is required in order to obtain a ceramic tile having a water absorption rate of up to 5%. For the purpose of decreasing the firing temperature, the usual practice is to use limestone. When using limestone, however, it is difficult to obtain a ceramic tile having a low water absorption rate of up to 5%. In order to reduce the firing temperature of the ceramic tile and the water absorption rate of the ceramic tile, therefore, it is necessary to use a raw material containing much alkaline elements, such as an alkali, an alkaline metal and an alkali earth metal.
Borax, which contains much alkaline elements, is a raw material present in large quantities in the natural world and stably available. The chemical composition of borax is not however constant. It is therefore the common practice to use a frit of borax obtained by melting borax and solidifying the molten borax (hereinafter referred to as the "frit"). The frit has a stable chemical composition and is not susceptible to degeneration.
However, the raw material cost of ceramic tiles is now increasing as a result of the shortage and the resulting price increase of the above-mentioned raw materials of ceramic tiles. There has therefore been a demand for the development of a new raw material for a ceramic tile other than those described above. With such circumstances in view, studies were carried out to consider the use of ash, which is obtained by separating sludge from sewage, and burning the separated sludge into a sludge ash, as a sintering agent in place of the above-mentioned feldspar and borax.
The sludge ash is not however stable in the chemical composition thereof and has a high melting point. The reason is that the sludge ash has a low ratio (CaO/SiO.sub.2) of the calcium oxide (CaO) content to the silicon dioxide (SiO.sub.2) content (this ratio of CaO/SiO.sub.2 is hereinafter referred to as the "basicity"). When using the sludge ash as a sintering agent for manufacturing a ceramic tile, therefore, the firing temperature of the ceramic tile cannot be reduced. For this reason, the sludge ash is not suitable, as it is, as a raw material for a ceramic tile.